This invention is in the field of steam generation using solar energy. In particular, a preferred embodiment of the present invention describes a point focus distributed receiver for generating high-pressure steam.
Solar energy has been used in numerous ways and for numerous purposes almost from the beginning of recorded human history. From the passive solar energy absorption/radiation of an Indian adobe building which kept the building cool in the daytime and warm at night to the photoelectric solar panels on a telecommunications satellite which provide electricity to its circuits, solar energy has been used in innumerable ways for innumerable purposes.
As our society has learned the environmental costs of producing electricity from both fossil and nuclear fuels, more attention has been turned to the possibility of producing electricity using solar powered generating systems. At first glance, solar energy seems to offer a free energy source, the sun, with no or very low environmental costs.
Such a superficial assessment has proven to be completely wrong. Although the energy source itself is free and non-polluting, it is not continuously available, given the vagaries of clouds and seasons, not to mention the day/night cycle. In most regions, solar energy is not of very great power density, necessitating very large solar energy collectors. To make the generation of electricity from solar power cost competitive with known fossil fuel generators and nuclear power plants, the solar energy collectors must be designed to be inexpensive.
One known method of creating electricity from solar energy is the use of photoelectric cells. Sunlight falling on the semiconductor material of the cell generates free electrons in the material, thereby creating an electric current. Although such cells are conceptually simple, their manufacture is costly, requires a great deal of energy, generates a large amount of toxic waste and the energy conversion efficiency of the resultant cells is low. Attempts to increase the efficiency of these cells have been somewhat successful, but at the cost of using more exotic materials, further increasing the cost of the cells. At present, using photoelectric cells to generate electricity is not practical on a large scale, although they are used in remote areas and in outer space where conventional energy sources are unavailable.
Other systems for generating electricity using solar energy require a concentrator and a receiver, the concentrator intercepting the solar radiation and focusing it on the receiver, the receiver absorbing the focused solar radiation, convening the radiation to heat and transferring the heat to a working fluid. For a system to generate electricity, the working fluid must be heated to a high temperature. In the past several decades, many designs for high temperature solar collectors (the combination of the concentrator and receiver) have been proposed and numerous prototypes have been built.
These various designs of high temperature solar energy collectors can be broadly grouped into four categories. The first category is the trough collector, in which a trough, typically shaped as a parabola, focuses energy on a pipe that runs the length of the trough, the pipe being fixed at the focal point of the parabola. The trough is driven along one axis to track the sun. The second category is a fixed mirror, tracking receiver ("FMTR") system which uses a fixed spherical mirror as the concentrator. In an FMTR system, the receiver is suspended from the center of the sphere defined by the mirror and driven in two axes to track the image of the sun through its daily and seasonal motions. A third category, the central receiver, which is also known as the power tower, uses an array of mirrors, which mirrors are nearly flat, mounted around a tower, the tower having a single receiver. The mirrors are driven to follow a point halfway between the sun and receiver, as seen by each mirror, so that the solar image is kept focused on the receiver. The fourth category is a point focus, distributed receiver system. In this system, each concentrator has a receiver mounted on it, the receiver being fixed at the mirror's focus point. The receiver/mirror combination is driven to follow the sun through its daily and seasonal motions. Depending upon the amount of power that is required, a power generation system using point focus distributed receivers could comprise either one or a plurality of such collectors.
The common failing of these known solar energy systems is that they have not been cost competitive with fossil fuel heat generation. The economics of nuclear energy generation are beyond the scope of this patent disclosure. However, no new nuclear energy plant has been contracted for in the last five years. Consequently, the cost competitor for solar energy remains fossil fuels. In particular, the design of known solar systems has not dealt with the problems faced by such systems in a cost effective way.
One of the problems that solar energy generation systems face is the wind loading that the system must endure. Wind speeds can, on rare occasion, reach 50 m/s in most places suitable for solar energy production. Even higher wind speeds have been recorded in the hurricane belt. One proposed Department of Energy solar generation system would use collectors with a 15 m diameter. Such a collector, if it were required to meet ANSI standards for buildings (A58.1-1982), would have to survive a total force of 350,000N. Wind pressure is a major concern in the structural design of buildings, but it is an overwhelming concern in the design of moving solar collectors.
Most known solar collectors are built and mounted on monopods which are driven in altitude and azimuth. For large collectors, the monopod and its pier become very large and expensive. Also, the monopod can potentially interfere with the mechanical bracing of the concentrator dish itself. In high winds, the monopod and its attached concentrator dish must be driven to a stowed position quickly, which requires both a large motor and a large gearing system. Finally, monopods do not lend themselves to simple solar tracking mechanisms.
Another problem with known solar energy generation systems involves the working fluid, which can comprise a molten metal such as sodium, a liquid salt such as sodium chloride, various oils, or water. The working fluid must carry the thermal energy to the heat engine or process. Generally the fluid is both hot and at high pressure. As the solar collector moves, the thermal load remains fixed. Delivery of the solar generated heat to the thermal load thus requires a slip joint or a flexible coupling. In known systems, leaks at these joints and flexible couplings has proven to be a major problem. In some cases, the thermal load, generally a heat engine, is mounted in the immediate vicinity of the focal point of the concentrator, avoiding the need for plumbing to a stationary load.
In all solar energy systems, heat loss is a major problem. In trough collector systems, the advantage of a simpler, single axis drive system is more than offset by the large thermal losses of the system. Although heat loss is a particular problem in trough systems, it is present to some degree in all solar energy systems, reducing their overall efficiency and increasing their cost.
The combination of drive system complexity, fluid loss, thermal losses, and the expense of both system construction and site purchase has made solar energy as a means to generate electricity impractical. Without numerous improvements in known solar energy generation systems, the production of electricity by means of solar energy will not be cost-competitive with fossil fuels.